In the conventional rack gear manufacturing techniques, one lateral side of a rack gear is machined in a regular pitch to form a teeth portion. However, these conventional methods embrace many problems due to high-cost and lower efficiency.
On the other hand, recently a liner gear of non-regular pitch has been used and a variable gear ratio (VGR) has been applied. In machining a rack gear having a non-regular pitch, a technology with high precision is required, and a high cost and extended process time are disadvantageously demanded as well.
In recent years, therefore, a forging method has been used to form the teeth of a rack gear in order to mitigate the demerits in the machining technique. The forming method has an advantage that a desired type of teeth can be formed in a relatively short time by plastic deformation of one lateral side of a linear bar. In addition, the mechanical properties of the rack gear can be improved.
For example, Japanese Patent Laid-Open Publication No. 2004136369 and U.S. Pat. No. 6,289,710 disclose a press forging for forming the teeth of a rack gear.
In the above mentioned press-forging technique, however, extremely small portion of a linear bar can be formed with teeth. Furthermore, the technique has drawbacks that distortion occurs quite often during plastic deformation and brittleness of the rack gear increases by instantaneously applying a high load deformation to the linear bar.